score = input("请输入学生分数：")

if not score.isdigit(): # 判断输入是否为数字
    print("输入错误！")
else:
    score = int(score)
    if score >= 90 and score <= 100:
        print("A")
    elif score >= 80 and score < 90:
        print("B")
    elif score >= 60 and score < 80:
        print("C")
    elif score >= 0 and score < 60:
        print("D")
    else:
        print("输入错误！") 

